1. Field of the Invention
The present invention relates to methods and apparatus for selecting, among successive signals, those which are spaced from each other by more than a predetermined time interval. The invention is particularly useful for enhancing the resolution in pulse analysis by rejecting signals which are distorted not only by overlapping or pile-up effect but also by chopping at the limits of gate intervals during which the signals are sensed.
2. Description of the Prior Art
Various methods and devices have been proposed for rejecting signals which are too close to each other. In the most commonly used method signals to be selected trigger a monostable, which delimits a "dead" time interval immediately following each signal. If a second signal occurs during the "dead" time interval initiated by a first signal, it will be rejected. As shown schematically in FIG. 1, signals to be selected follow two separate channels. In the first channel, signal are amplified, suitably delayed and applied to the signals input of a linear gate which forms the common end of the two separate channels. In the second channel, signals are first differentiated by a filter-amplifier and then each differentiated pulse triggers a first monostable, which starts a "dead" time interval T delimited by a square-wave signal. The differentiated pulses are applied to one of the two inputs of an AND-gate, while the other input receives the square-wave signals. Signals at the output of the AND-gate are applied at the input of a second monostable starting a square-wave signal of width 2T. A square-wave signal 2T is obtained at the output of the second mono-stable only if a second signal has occurred within the "dead" time interval T initiated by a first signal. The square-wave signal delivered at the output of the monostable 2T is then inverted to obtain an inhibit signal which is applied to the control input of the linear gate common to the two separate channels followed by the signals to be selected. A validation signal level is present at the control input of the linear gate only when there is no square-wave inhibit signal at the output of the monostable 2T, and so only if the signal at the signal input is outside the "dead" time intervals of length T. Consequently, a signal is obtained at the output of the linear gate only if this signal is separated from the preceding signal by a time interval larger than the "dead" time interval T.
Devices of the prior art work satisfactorily when the frequency of repetition of signals is not very high, so that only pairs of closely spaced pulses need be considered and the occurrence of triplets, quadruplets and higher orders of closely spaced pulses can be ignored. Moreover, the components used in the apparatus of the prior art, especially the monostable device, are not reliable when submitted to large changes of environmental conditions. Particularly, the length of the "dead" time intervals can vary to a non-negligible extent as a function of temperature.
Accordingly, it is an object of the present invention to provide new methods and apparatus for selecting, among signals, those which are spaced from one another by more than a predetermined time interval.
Another object of the invention is to provide methods and apparatus for selecting among input signals, during preselected time periods, those input signals which are spaced from each other and from the limits of said time periods by more than a predetermined time interval.